When an oil well is first drilled and completed, the fluids (such as crude oil) may be under natural pressure that is sufficient to produce on its own. In other words, the oil rises to the surface without any assistance.
In many oil wells, and particularly those in fields that are established and aging, natural pressure has declined to the point where the oil must be artificially lifted to the surface. Subsurface, or downhole, pumps are located down in the well below the level of the oil. A string of sucker rods extends from the pump up to the surface to a pump jack device, or beam pump unit. A prime mover, such as a gasoline or diesel engine, or an electric motor, or a gas engine on the surface causes the pump jack to rock back and forth, thereby moving the string of sucker rods up and down inside of the well tubing.
The string of sucker rods operates the subsurface pump. A typical pump has a plunger that is, reciprocated inside of a barrel by the sucker rods. The barrel has a standing one-way valve, while the plunger has a traveling one-way valve. In some pumps, the plunger has a standing one-way valve, while the barrel has a traveling one-way valve. Reciprocation charges a compression chamber between the plunger and the barrel valve with fluid and then lifts the fluid up the tubing toward the surface.
In gaseous wells, gas, such as natural gas, is produced with the liquid oil (and in many wells, water). In gaseous wells, the compression chamber should be as small as possible, in order to increase the compression of the gas to a sufficient pressure so as to open the traveling valve in the plunger. Failing to sufficiently compress the gas results in gas lock, wherein no fluid is pumped.
For a fixed barrel pump, the size of the compression chamber is determined by locating the bottom of the plunger close to the barrel standing valve, when the plunger has reached its lowermost position. The lowermost position of the plunger is determined by a stop, which stop is formed by a guide secured to the top of the barrel. The guide contacts a bushing that is connected by way of a valve rod or pull tube to the plunger. In prior art pumps, the bottom of the plunger is positioned above the barrel standing valve, and thus the size of the compression chamber is determined, by selecting a valve rod or pull tube of suitable length.
The prior practice requires stocking a large number of different sizes of valve rods and pull tubes. Furthermore, prior art practice frequently requires a valve rod or pull tube to be shortened by cutting and possibly rethreading the cut end.
Still another problem with the prior art practice is the inaccuracies introduced into the pump. Sometimes, the guide is tightened onto the barrel after the valve rod or pull tube has been cut to length, resulting in an improperly sized compression chamber.